Many telecommunications devices such as routers, bridges and switches are comprised of a frame of a telecommunication device and plug-in units installed in plug-in unit slots in said frame. Electric connectors in a plug-in unit make galvanic contacts with corresponding electric connectors in the frame when the plug-in unit is inserted in the plug-in unit slot in said frame. The frame may have wirings such that plug-in units installed in said frame form a full mesh network or such that plug-in units installed in said frame are connected to a central element of the telecommunications device which may comprise one or more plug-in units installed in said frame or which may be a functional component integrally built in said frame.
Certain care is needed in the installation of plug-in units in the frame of a telecommunications device because the galvanic contacts between the electric connectors of the plug-in unit and corresponding electric connectors of the frame may be poor, or even non-existent, if the plug-in unit is not properly seated in its place. In a prior-art solution, some pins in the electric connectors between the plug-in unit and the frame are shorter than other pins in said electric connectors. Those shorter pins are part of a test circuit for detecting whether the plug-in unit is installed in the plug-in unit slot in such a manner that said shorter pins make galvanic contacts in the electric connectors between said plug-in unit and said frame. If said shorter pins make the galvanic contacts it is practically certain that the longer pins in said electric connectors make adequate galvanic contacts in the electric connectors. The above-described prior-art arrangement for indicating correct installation of a plug-in unit is possible if the frame of the telecommunications device has got wirings belonging to the test circuit mentioned above. Often, however, there are no such wirings in the frame of a telecommunications device and the post-installation of such wirings would be expensive and difficult especially if said telecommunications device has been delivered to the telecommunications operator and is already in productive use.